Silver halide photothermographic imaging materials, often referred to as "dry silver" compositions because no liquid development is necessary to produce the final image, have been known in the art for many years. These imaging materials typically comprise a light insensitive, reducible silver source material; a light sensitive material which generates silver when irradiated; and a reducing agent for the silver ion in the silver source material.
The silver source material is a material which contains silver ions. The earliest and generally preferred silver source materials comprise silver salts of long chain carboxylic acids, usually of from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of like molecular weight have primarily been used.
The light sensitive material is typically a photosensitive silver halide which is in catalytic proximity to the light insensitive silver source material. Catalytic proximity is an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photosensitive silver halide, those nuclei are able to catalyze the reduction of the silver source by the reducing agent.
In these photothermographic imaging materials, exposure of the silver halide to light produces small clusters of silver atoms. The imagewise distribution of these clusters is known in the art as the latent image. This latent image generally is not visible by ordinary means and the light sensitive article must be further processed in order to produce a visible image. The visible image is produced by the catalytic reduction of the silver ions of the silver source material which are in catalytic proximity to the silver specks of the latent image.
Color-forming, "dry silver" imaging systems are likewise well known in the photothermographic art. Color formation is typically based on the silver catalyzed oxidation/reduction reaction between the silver source material and the reducing agent. Typically, the reducing agent is a colorless or lightly colored leuco dye or dye forming developer that is oxidizable to a colored state.
Multicolor photothermographic imaging articles typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsion layer. Photothermographic articles having at least 2 or 3 distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4,021,240 and 4,460,681.
Typically each of the color-forming photothermographic emulsion layers contains a reducible silver source material, a spectrally sensitized photosensitive silver halide, a reducing agent for silver ion and a solvent soluble binder. For example, U.S. Pat. Nos. 4,460,681 and 4,452,883 disclose multicolor photothermographic articles in which each photothermographic emulsion layer is sensitized to a portion of the spectrum at least 60 nm different from the other photothermographic emulsion layers, and each photothermographic emulsion layer contains a leuco dye which when oxidized forms a visible colored dye having a maximum absorbance at least 60 nm different from that of the dye formed in the other photothermographic emulsion layers. Usually one of the color forming photothermographic emulsion layers forms a yellow color. Although such multicolor photothermographic imaging materials are well known in the art, in recent times considerable effort is being expended to increase the stability of the emulsions and decrease the time and temperature required for thermal development. However, such efforts have often encountered the traditional problem of balancing the development rate of the emulsion with the shelf-stability of the photothermographic article. The more rapidly the image may be developed in the emulsion during thermal development, the greater the tendency the emulsion has to form dyes without exposure and heating. As a result, conventional methods of speeding up the rate of color formation, such as by using fast coupling color couplers or easily oxidizable leuco dyes in the photothermographic system, consistently tend to increase the formation of spurious dye images (i.e., background coloration or fog).
As a solution to this problem, compounds are continually being sought which decrease the time and temperature required for development of the photothermographic emulsion without lessening the stability of the photothermographic article or the quality of the image produced. In this respect U.S. Pat. Nos. 4,626,500; 4,629,684; and 4,640,892 disclose development accelerator compounds for use with photothermographic emulsions containing a silver halide, a leuco dye and an organic silver salt oxidizing agent. Purportedly these compounds provide a heat developable color photographic light sensitive material which provides an image having a high maximum density and a low fog by heat developing at a relatively low temperature and for a relatively short time.
The time and temperature required for the thermal development of multicolor photothermographic articles are typically determined by the time and temperature required to develop the color-forming emulsion layer having the slowest development rate. In multicolor photothermographic articles having a yellow-forming emulsion layer, it is generally the yellow forming emulsion layer which requires the longest development time and/or the highest development temperature to achieve sufficient image density. It is toward the end of reducing the time and/or temperature required to thermally develop a yellow-forming emulsion layer that the present invention pertains.